1. Field of the Invention
The invention relates generally to gamma-ray logging in a borehole.
2. Background Art
Measuring gamma-rays with a detector located within a borehole is a common operation in well logging.
Natural gamma-rays are emitted in a decay of subsurface materials such as thorium, uranium and potassium (Th, U, K), each of which emits a characteristic spectrum resulting from an emission of gamma-rays at various energies. The measuring of the natural gamma-rays is particularly useful in the exploration for exploitation of oil and gas resources because it is believed that the concentrations of Th, U, K taken individually or in combination are a good indication of previously unavailable information as to the presence, type and volume of shale or clay in the formations surrounding the borehole.
A detector in a spectral mode, i.e. a detector that is sensitive to the energy of the gamma-rays, may provide a gamma-ray spectrum as a function of energy.
Alternatively, the gamma-rays may be counted without any energy discrimination: such a raw counting mode provides useful information about a presence of shale.
Furthermore, a gamma-ray detector may also detect neutron-induced gamma-rays. Using a neutron source in a logging tool for obtaining a characteristic of a formation surrounding a borehole is well known, particularly for measuring a formation porosity.
Certain techniques involve the use of a chemical source such as AmBe to provide neutrons to irradiate the formation such that scattered neutrons returning to the borehole may be detected and the formation characteristic (porosity) inferred. The irradiating of the formation may also induce gamma-rays from a decay of excited atoms that may be detected by the gamma-ray detector.
The neutron source may be an electronic generator of neutrons, which allows to irradiate the formation with neutrons having a much higher energy (14 MeV) than an average energy of the neutrons generated by a traditionally used AmBe source (4 MeV). As a consequence, there is a significant increase in a number of nuclei in the formation, which are transmuted into radioactive elements.
In particular, oxygen nuclei may be converted into nitrogen nuclei; the radioactive nitrogen atoms decay quickly by beta decay to an excited state of oxygen, which in turn decays by emitting gamma-rays. A majority of the emitted gamma-rays have an energy around 6.1 MeV, which is much higher than gamma-ray energies from naturally occurring radioactive materials.
A gamma-ray detector may also count gamma-induced gamma-rays produced by a gamma-ray generator. The gamma-ray generator irradiates the formation with gamma-rays having a relatively low energy, e.g., 600 keV. The gamma-rays are scattered by electrons in the formation, losing energy at each scattering event. The scattered gamma-rays hence also have a relatively low energy, and may be detected at the gamma-ray detector to provide information about the formation surrounding the borehole.
Radioactive tracer gamma-rays may also be detected at a gamma-ray detector. A radioactive tracer is injected into the formation and/or the borehole and emits radioactive tracer gamma-rays. The radioactive tracer gamma-rays are detected and provide information about a possible behavior of fluids within the formation and/or behind a casing.
It is hence possible to detect within the borehole gamma-rays from a plurality of sources.
A gamma-ray logging may be performed during a drilling operation of the borehole, so as to provide information about the formation surrounding a drilled portion of the borehole as soon as possible. FIG. 1 shows a schematic of an example of a system for logging while drilling. A logging while drilling tool 108 comprises a drill bit 101 at an end of a drill string 103. The drill string 103 is used to drill a borehole 102. Logging tools (104, 105, 109) are disposed within the drill string 103, so as to allow a drilling mud to be carried through a mud channel 106. The drilling mud is pumped down to the drill bit 101, where it helps clear cuttings and bring them to the surface through an annulus between the drill string 103 and a formation 107.
One of the logging tools (104, 105, 109) may contain a neutron generator 104 that irradiates the formation 107 with high energy neutrons, so as to provide a mapping of the porosity of the formation 107. A gamma-ray detector 109 may be provided close to the neutron generator to measure gamma-rays induced by the generated neutrons.
Furthermore, a gamma-ray detector 105 may measure the natural gamma-ray activity of the formation 107. The gamma-ray detector 105 intended to measure the natural gamma-ray activity, may also detect gamma-rays produced by a gamma-ray inducing source, e.g. the neutron generator 104.
A correction method for a detector intended to detect gamma-induced gamma-rays is described in U.S. Pat. No. 5,459,314. A density source irradiates a formation with gamma-rays that interact with the formation and are detected after being scattered in a formation or a borehole. The detector intended to detect the scattered gamma-rays may also detect non-gamma induced gamma-rays which are not related to the gamma-rays emitted by the density source, i.e. gamma-rays that are generated by another logging tool source or natural gamma-rays from the formation. The correction method consists in identifying and removing the detected non-gamma induced gamma-rays. The identifying may be performed by detecting gamma-rays above a threshold energy level, and by determining a count of non-gamma induced gamma-rays. The count of non-gamma induced gamma-rays is then subtracted from a total gamma-ray count so as to obtain a count of gamma-rays from the density source.
A gain of the gamma-ray detector is defined as a ratio of an amplitude of gamma-ray signals and the energy of the gamma-rays. The gain of a gamma-rays spectroscopy system may vary as a function of an high voltage of a photomultiplier of the gamma-ray detector, the age of the photomultiplier, the temperature etc. It is hence necessary to stabilize the gain of the gamma-ray detectors.
A first method that is commonly used for stabilizing the gain consists in generating a peak of gamma-rays having a predetermined energy, i.e. a calibration peak having a predetermined position that is well defined. Since the predetermined energy of the calibration peak is known, it is relatively easy, once the calibration peak is detected at the detector, to adjust the gain so that a detected position of the calibration peak equals the predetermined position. Such a method may be implemented with a basic detector and three discriminators. The discriminators are used to detect gamma-rays within two energy windows. However, it is necessary, when counting relevant gamma-rays, to subtract gamma-rays of the calibration peak from a total of detected gamma-rays. Hence counting errors may be relatively high. For a detection of a natural gamma-ray activity where a counting rate may be relatively low, the counting of the relevant gamma-rays may not be precise enough if the first method is used.
A second method, described in U.S. Pat. No. 5,360,975, consists in recording a full gamma-ray spectrum and determining a best fit between a reference spectrum and the recorded spectrum. The gain of the best fit is used to regulate the gain of the gamma-ray detector. This method requires the detector to be in spectral mode to obtain the full gamma-ray spectrum.
European Patent EP0640848 describes a third method for use in a cased well. The third method aims at stabilizing the gain of a detector intended to be used for counting high-energy neutron-induced gamma-rays. A high-energy neutron generator irradiates the casing and the formation with high energy neutrons, which creates the high-energy neutron-induced gamma-rays and thermal-neutron-induced gamma-rays. Timing means are provided so as to obtain a measurement of both gamma-rays. As the casing contains iron atoms, an iron peak is always present in the measurement of the thermal-neutron-induced gamma-rays. The stabilizing of the gain of the detector is hence based on the iron peak. Once the gain is stabilized, it is considered that the measurement of the high-energy neutron-induced gamma-rays is correct.